The present invention relates to an optical connector using a photonic crystal that can be used for optically coupling an external optical system, such as an optical fiber, and an optical element using a photonic crystal, an optical coupling method, and an optical element incorporating the optical connector.
Photonic crystals composed of a periodic array of two or more materials of difference refractive indices are drawing attention because they can highly control the behavior of light. Photonic crystals allow light to be efficiently confined in a spatial domain that is no greater than the wavelength or to be refracted at a steep angle with low loss, so that the optical element can be significantly smaller than conventional. Confinement of light can be most efficiently achieved by using a three-dimensional photonic crystal. However, it is difficult for the existing processing art to precisely fabricate a fine periodic structure on the order of 1 μm, such as a wavelength range for optical communications, and thus, application of a two-dimensional photonic crystal to various devices is being contemplated.
For example, the periodic refractive index structure of a two-dimensional photonic crystal is composed of a high-refractive-index material, such as silicon, and a square lattice array or triangular lattice array of cylindrical holes (holes in the form of a cylinder) made of a low-refractive-index material, such as air, formed therein. Such a periodic structure provides a photonic band gap, and propagation of in-plane light is controlled. On the other hand, the two-dimensional periodic structure cannot control vertical propagation of light in a direction perpendicular to the in-plane direction. Thus, the two-dimensional periodic structure is made in a slab form, and layers of a low-refractive-index material, such as air, are provided above and below the two-dimensional periodic structure, thereby allowing vertical confinement of light by the total reflection due to the difference in refractive index.
If a line defect (formed by eliminating a row of cylindrical holes made of a low-refractive-index material) is formed in the periodic refractive-index structure of the two-dimensional photonic crystal slab, there can be provided an optical waveguide that can propagate light with low loss owing to the periodic structure in the in-plane direction and the total reflection in the vertical direction. Thus, it is expected that an ultra-compact optical integrated circuit can be provided by integrating optical functional elements, such as an optical waveguide and an optical filter, in a photonic crystal.
In order to put an optical element using a photonic crystal into practical use, the optical element has to be capable of being optically coupled to an external optical system, such as an optical fiber. As a light introducing section of the two-dimensional photonic crystal slab, the line defect optical waveguide described above is popular. As a method of optically coupling external light to the line defect optical waveguide, Japanese Patent Application Laid-Open No. 2001-272555 (issued on Oct. 5, 2001, referred to as literature 1, hereinafter) discloses an art of externally inputting light perpendicularly to a surface of a two-dimensional photonic crystal slab, thereby optically coupling the light to the slab surface.
Specifically, according to the method described in the literature 1, a point defect that disturbs the periodic refractive index arrangement is formed in the two-dimensional photonic crystal slab, and light is input to a line defect optical waveguide via the point defect or externally output from the line defect optical waveguide via the point defect. The point defect is formed by changing the diameter of a cylindrical hole (air hole) in the periodic refractive index structure, and light that can be input or output via the point defect is limited to light having a particular wavelength that depends on the shape (diameter) of the point defect. Thus, if a plurality of point defects having different shapes are formed in the photonic crystal, light of different wavelengths can be input to or output from the line defect optical waveguide via the respective point defects.
However, the method of coupling light perpendicularly to the slab surface via a point defect formed in the two-dimensional photonic crystal slab described in the literature 1 has a problem that the optical coupling efficiency is extremely low because the size of the point defect (having a diameter of about 0.5 μm, for example) is extremely smaller than the mode size (about 10 μm, for example) of the external optical system, such as an optical fiber. In addition, since the method is intended for selective coupling of only light of a single wavelength by controlling the shape of the point defect, there is a problem that light containing components of different wavelengths from one optical fiber cannot be coupled to the optical waveguide, for example. Furthermore, there is a problem that the optical fiber has to be accurately aligned with the point defect in the plane of the two-dimensional photonic crystal slab.
That is, there have not been proposed an optical connector or an optical coupling method that improve the optical coupling efficiency, which is reduced because of the difference between the size of the point defect and the light mode size of the external optical system, can couple a plurality of light components of different wavelengths from one optical fiber to the line defect optical waveguide in the two-dimensional photonic crystal slab, and can readily achieve alignment, that is, can maintain a constant optical coupling efficiency even if the relative position of the optical fiber with respect to the two-dimensional photonic crystal slab is shifted by several micrometers or more.